The depolymerization of polyethylene terephthalate by hydrolysis at a high temperature and pressure in the absence of a base or acid, or a catalyst, is known in the art, see U.S. Pat. Nos. 4,578,502; 5,605,762; G.B. Patent 2,123,403; U.S. Pat. No. 4,578,510; Japanese Patent 56118420; Czech Patent 169,292; Japanese Patent 49076968; and Jap. Kokai JP 49/41329. However, the depolymerization of polyethylene terephthalate by neutral hydrolysis can result in the production of oligomeric byproducts (U.S. Pat. No. 4,578,510); derivatives of terephthalic acid (Wlokma Chem., 13(2), 144-55); cyclic trimers (JP 56118420). Additionally, the depolymerization product of waste polyethylene terephthalate in the form of bottles, film, fiber and other manufactured articles can contain dyes and other contaminants. For example, 100 parts of deep blue polyester fabric before hydrolysis contained 6.5% dyes and contaminants (JP 49020147). Accordingly, although various processes are available for hydrolyzing PET waste, the purification of the terephthalic acid typically requires several steps to remove dyes, pigments, and other impurities including inorganic compounds such as catalyst residues and organic compounds which can result from the depolymerization reaction.
U.S. Pat. No. 4,355,175, to Pusztaszeri, exemplifies the difficulties encountered in preparing a purified terephthalic acid from waste PET. Polyester scrap such as film (plain or silver-bearing), fabric, yarn, or bottles, was depolymerized at room temperature with a mixture of concentrated sulfuric acid and water to form crude terephthalic acid. Pusztaszeri teaches that an alkaline solution, which can be dark brown, or black in color, containing the crude terephthalic acid resulting from the depolymerization, is filtered to obtain a clear liquid which may be light brown in color (if dark colored, it must be reacted with activated charcoal and refiltered from the charcoal). The obtained solution is then acidified with sulfuric acid to precipitate the terephthalic acid (TPA). The TPA is then filtered and washed.
Regardless of the method of depolymerization and the method of purification of the resulting TPA, the variable nature of the impure crude terephthalic acid resulting from the depolymerization of waste polyethylene terephthalate from many sources and the variable nature of the impurities resulting therefrom and contained in the crude terephthalic acid, the process control, and thus quality assurance of the purified terephthalic acid, has been made difficult and costly. Because of this lack of quality assurance and its cost relative to that of virgin purified terephthalic acid, purified terephthalic acid from waste polyethylene terephthalate has not been considered as a viable economic replacement for fiber grade virgin purified terephthalic acid prepared from paraxylene.
It therefore is an object of this invention to provide a process to prepare fiber grade purified terephthalic acid from waste polyethylene terephthalate wherein metal content (ash content) is less than about 100 parts per million by weight (ppmw), total organic impurities are less than 1000 ppmw, b*-value is less than 2 and relative fluorescence concentration in visible light (RFCVIS) is less than about 2500.
It is an object of this invention to provide an economical process for preparing fiber grade purified terephthalic acid from waste polyethylene terephthalate wherein origin of the waste terephthalate is from film (silver-bearing and non-silver bearing), fiber (including yarn and fabrics), bottles (clear or colored), and polyethylene terephthalate residues from manufacturing polyethylene terephthalate. Environmental problems relating to aspects of waste polyethylene terephthalate are reduced or eliminated.
Other objects of this invention will be obvious from the following discussion.